Masonite is another possibility, but it also disintegrates in moisture and is rather heavy. PVC can be bought in sheets at hobby stores, but PVC can shatter. You can get ABS in sheet form at different thicknesses, and it's shatter-resistant, but you may have a hard time finding reasonable sized sheets except online, when you might have to buy a huge 4x8' sheet. I would suggest checking a framing store for shatter-resistant plastic used in picture frames (probably acrylic). This is most important for a panel to protect the front, especially the screen. Even if you use some kind of folder like a day planner, you might want to tuck a rectangle of shatter-resistant plastic (or sealed hobby board, as Ricky suggests) into the front to protect the screen.

MAKE magazine had an article about making an iPod case from carbon fiber and resin in Volume 9. You might find it helpful for your purposes, though I wouldn't rely on fiber and resin to protect the screen.